Alkylene glycols, in particular monoalkylene glycols, are of established commercial interest. For example, monoalkylene glycols are being used in anti-freeze compositions, as solvents and as base materials in the production of polyethylene terephthalates e.g. for fibers or bottles.
The production of alkylene glycols by hydrolysis of alkylene oxides is known, both by liquid phase hydration of alkylene oxides with an excess amount of water, e.g. of 20 to 25 moles of water per mole of alkylene oxide, or by hydration in a heterogeneous system. The reaction is deemed to be a nucleophilic substitution reaction, whereby opening of the alkylene oxide ring occurs, water acting as the nucleophile. Because the primarily formed monoalkylene glycol likewise acts as nucleophile, as a rule a mixture of monoalkylene glycol, dialkylene glycol and higher alkylene glycols is formed. In order to increase the selectivity to monoalkylene glycols, it is necessary to suppress the reaction between the primary product and alkylene oxide, which competes with the hydrolysis of alkylene oxide.
One effective means for suppressing the secondary reaction is to increase the relative amount of water present in the reaction mixture. Although the selectivity with respect to the monoalkylene glycol is thus improved, a problem consists in that for the recovery of the monoalkylene glycol from the reaction mixture, large amounts of water have to be removed. This is usually done by evaporation, which is followed by distillation of the desired product from the evaporation residue. It will be understood that the separation of large amounts of water from the product involves large expenditure and is economically unattractive.
Considerable efforts have been made in order to achieve an increase in selectivity with respect to the monoalkylene glycols, without having to use a large excess of water. Usually, these efforts have focused on the selection of more active hydration catalysts and in fact there are many publications, in which results obtained with various types of catalysts are disclosed.
Both acid and alkaline hydration catalysts have been investigated, whereby it would appear that the use of acid catalysts enhances the reaction rate without significantly affecting the selectivity, whereas by using alkaline catalysts generally lower selectivities with respect to the monoalkylene glycol are obtained.
In U.S. Pat. No. 4,393,254, a process for the hydration of alkylene oxides is described whereby use is made of partially amine-neutralized sulfonic acid resins as catalysts, for example Amberlyst XN-1010, neutralized for 50% with triethylamine. Although these catalysts allow low water/alkylene oxide ratios, the obtainable conversions are unsatisfactory, typically about 70%.
Higher conversions can be obtained with the process, as disclosed in EP 156,449. According to this document, the hydrolysis of alkylene oxides is carried out in the presence of a selectivity-enhancing metalate anion-containing material, preferably a solid having electropositive complexing sites having affinity for the metalate anions. The said solid is preferably an anion exchange resin, the metalate anions are specified as molybdate, tungstate, metavanadate, hydrogenpyrovanadate and pyrovanadate anions. It is considered that an adduct is formed between the metalate anion and the alkylene oxide, which adduct is subsequently hydrolyzed to form the alkylene glycol. The formation of the adduct competes with the reaction between monoalkylene glycol and alkylene oxide which would result in the formation of di- and trialkylene glycols. The selectivity with respect to monoalkylene glycols is thus enhanced, without having to supply excessive amounts of water. However, a complication of this known process consists in that the alkylene glycol-containing product stream also comprises a substantial amount of metalate anions, displaced from the electropositive complexing sites of the solid metalate anion-containing material. In order to reduce the amount of metalate anions in the alkylene glycol product stream, this stream is contacted with a solid having electropositive complexing sites associated with anions which are replaceable by the said metalate anions.
It has been proposed to simplify the product recovery procedure by using water-insoluble vanadate and molybdate salts. However, with these metalate anion salts the obtained selectivities are significantly lower than with the water-soluble metalates.
EP-A-226,799 discloses a method for preparing ethylene glycol and/or propylene by hydrating the respective alkylene oxide in the presence of a catalytic combination of a carboxylic acid and a salt of a carboxylic acid, both of which may be used in an arbitrary combination. These acid/salt combinations are in solution, which makes their separation from the reaction product necessary.
JP-A-57-139026 discloses a method for reacting alkylene oxide with water in the presence of a halogen type anion exchange resin and in the co-presence of carbon dioxide.
RU-C-2001901 points out that the former disclosure has the disadvantage of the formation of carbonates in the reaction mixture, which are difficult to separate from the glycols on account of the closeness of their boiling points. This patent publication discloses as its invention the performance of the alkylene oxide hydrating reaction in one or in a sequence of `extrusion reactor(s)` (continuous reaction), in the presence of `anionite` (anion exchange resin of the quaternary ammonium type) in bicarbonate form and carbon dioxide. The essential difference with the former Japanese patent publication appears to be the use of the bicarbonate from of the anion exchanger instead of the halogen form thereof. And yet, the Russian patent does not dispense with the addition of carbon dioxide to the feed.
It has now been found that the preparation of alkylene glycols proceeds with high conversions and selectivities to the monoalkylene glycols, by performing the reaction with the aid of a specific catalyst composition, substantially free of metalate or halogen anions.